Rasberry Pis and similar are sometimes sold as credit-card sized, but they are not portable, really, have huge Ethernet ports and no display or keyboard. Those are on the too-big side for what i am looking for.
In the other hand, you could say that SIM cards or USB devices are technically computers, but they lack a user interface. They are too small.
Is there something small with a usuable display and input method; something that looks like a calculator or tiny smartphone? Does not need a battery (although that’s a plus). Does not need WiFi or 5G (although that’s plus). Needs some standard method to communicate with the outside world (USB, Bluetooth, NFC, Wifi, …). Should easily fit in a pocket, wallet, purse.
Ideally something that can run a minimal Linux or Android?
They are not intended to be a finished product, you are supposed to add your own display and keyboard. If you want really portable start with the Pi zero which doesn’t have the big ports, then slap on a small display and keyboard of your choice. There are small kits like these https://ameridroid.com/collections/all-products/products/odroid-go-advance for example. There are also a few different ones with blackberry keyboards https://liliputing.com/beepberry-is-a-79-hackable-pocket-computer-kit-with-a-blackberry-keyboard/ https://arstechnica.com/gadgets/2023/05/its-a-raspberry-pi-a-blackberry-keyboard-and-a-battery-its-the-beepberry/ (when blackberry quit making hardware they surplussed a bunch of keyboards)
Cool idea with the bb Keyboards.